The present invention relates generally to magnetic materials, and particularly to layered bimetallic structures exhibiting perpendicular magnetic anisotropy.
A major objective of research efforts seeking to improve magnetic recording media has been to increase the absolute density of recorded information. Practically all magnetic recording media now in use employ a magnetizable layer or layers comprising magnetizable particles dispersed in an organic binder. During recording, these media are exposed to fields which magnetize the recording layer in a longitudinal, or in-plane direction, hereinafter referred to as the "parallel" direction. As recording density is increased in the parallel direction, however, the demagnetizing field of the recording layer increases, which results in reduction and rotation of remanent magnetization and a corresponding reduction in the output or playback signal. For these reasons, achievable information density in parallel recording systems is now reaching theoretical limitations.
A new approach to ultrahigh recording densities is known as perpendicular magnetic recording. In perpendicular magnetic recording modes, magnetization is applied to a recording medium along an axis normal, or perpendicular to, the plane of the recording layers. In this mode, as the wavelength of recording signals is decreased and recording density is increased, the demagnetizing field is decreased. Moreover, where adjacent bits are oppositely magnetized, they tend to reinforce, rather than demagnetize each other, further contributing to higher recording densities. Iwasaki, IEEE Trans. Magn. MAG-20:657 (1984) provides a discussion of the state of the art relating to perpendicular magnetic recording media.
Perpendicular magnetic recording modes require a recording medium having an "easy" axis of magnetization which is normal, or perpendicular, to the plane of the recording layer. The majority of magnetic recording media tend to favor an "easy" axis of magnetization which is parallel to the plane of the recording layer. However, a few candidate materials have been investigated which exhibit perpendicular magnetic anisotropy as a result of their crystallographic characteristics.
For example, certain ferrites, notably barium ferrite, exhibit a hexagonal crystal structure which can be exploited to achieve perpendicular magnetic anisotropy. These materials can be applied to substrates by sputtering, as disclosed by Naoe et al., IEEE Trans. Magn. MAG-17:3184 (1981), or exposed to a strong magnetic field to align the c-axis of the ferrite crystallites normal to the plane of the layer in which they are applied. Exemplary of the latter approach is the method for preparing perpendicular recording media disclosed by Oguchi et al., U.S. Pat. No. 4,447,467. According to this method, hexagonal crystallites of barium ferrite, strontium ferrite, lead ferrite, or calcium ferrite are dispersed in a fluid coating layer on an inert substrate. Following exposure of the resulting composite to a magnetic field to orient the crystallites in the perpendicular direction, the viscosity of the coating layer is increased to preclude further movement of the crystallites.
Cobalt also exhibits a hexagonal crystalline structure, and various alloys and other mixtures containing cobalt have been proposed for perpendicular magnetic recording.
For example, Kostenmaki, U.S. Pat. No. 4,472,248, discloses a method for preparing thin-film perpendicular magnetic recording media by electro-depositing a coating onto an electrically conductive substrate from an aqueous coating bath including cobalt, hypophospite, and optionally nickel ions.
Kitahara, et al., U.S. Pat. No. 4,452,864, describe perpendicular magnetic recording media having a magnetic layer composed of from 75 to 90 percent by weight cobalt, up to 15 percent molybdenum, and the balance vanadium and unavoidable impurities. These media are prepared by sputtering.
The most widely reported and apparently successful perpendicular magnetic recording media are sputtered films of cobalt-chromium alloys, which exhibit crystallographic perpendicular magnetic anisotropy. The role of chromium in such alloys is to reduce overall magnetization, and hence, in-plane demagnetizing forces. For example, Sugita, U.S. Pat. No. 4,429,016, discloses magnetic recording media produced by sputtering thin layers of a cobalt-chromium alloy onto a substrate. Additional details regarding cobalt-chromium perpendicular recording media can be found in Iwasaki, et al., IEEE Trans. Magn. MAG-14:849 (1978), and Kobayashi et al., J. Appl. Phys. 52:2453 (1981).
It has now been found that layered coherent structures of palladium and/or platinum and cobalt, characterized by a periodic structure comprising oligatomic layers of cobalt separated by regions of palladium and/or platinum, exhibit characteristics which are useful for magnetic recording, particularly perpendicular magnetic recording.